In technical usage, MIMO stands for “Multiple Input Multiple Output,” and refers to a system or method according to which multiple transmitting antennas and multiple receiving antennas are used for achieving an effect, for example for transmitting information, or, as in the present case, for detecting at least the position of an external object, using a radar device. Transmitting antennas are designed in particular for emitting an electromagnetic signal. Receiving antennas are designed in particular for receiving an electromagnetic signal. When reference is made to “antennas” here and in the following discussion, this is understood to mean transmitting antennas as well as receiving antennas, without distinction.
Numerous electronic systems are employed in recent vehicles, and may be used, for example, to assist a driver while driving the vehicle. For example, brake assist systems may recognize preceding road users and appropriately decelerate and accelerate the vehicle so that a predefined minimum distance from the preceding road users is always maintained. Such brake assist systems may also initiate an emergency brake application when they recognize that there is too little distance from the preceding road user.
To be able to provide such assistance systems in a vehicle, it is necessary to collect data concerning the surroundings of the particular vehicle. In the above-mentioned example of a brake assist system, it is necessary, for example, to detect the position of a preceding road user in order to be able to compute the distance of the vehicle from the preceding road user.
In the detection of the position of a preceding road user, for example the azimuth angle of the preceding road user, starting from the travel direction of the particular vehicle, may be detected. The azimuth angle is part of an advantageous spherical coordinate system based on the radar device, but also based on the vehicle for a radar device situated in a vehicle. The azimuth angle is situated with respect to the vehicle in such a way that for a variation from 0° to 360°, the azimuth angle covers a plane in parallel to the driving plane on which the vehicle is traveling. With the aid of the azimuth angle, for example an object to the left of the vehicle in the forward travel direction is distinguishable from an object to the right of the vehicle in the forward travel direction.
Since functionally irrelevant objects such as manhole covers or bridges also reflect electromagnetic signals as radar signals, detecting the elevation angle allows a distinction to be made between functionally relevant and irrelevant objects. The elevation angle is another part of the described spherical coordinate system, and for a variation from 0° to 360°, covers a plane perpendicular to the roadway on which the vehicle is traveling. The last independent coordinate of the described spherical coordinate system is the distance or radius.
Detection of the azimuth angles or elevation angles of an object may take place, for example, by evaluating the electromagnetic phase shifts of electromagnetic signals reflected on the object which are received at the receiving antennas of a radar device.
U.S. Published Patent Appln. No. 2012/256795 provides one possible antenna for such a radar device.
For a two-dimensional antenna array having phase centers xi in a first coordinate direction and yi in a second coordinate direction perpendicular thereto, the following expression is valid for phase φi at antenna i:
            φ      ⁢                          ⁢      i        =                            2          ⁢          π                λ            ⁢              (                              xi            *            sin            ⁢                                                  ⁢            θ            *            cos            ⁢                                                  ⁢            Φ                    +                      yi            *            sin            ⁢                                                  ⁢            Φ                          )              ,where θ represents the azimuth angle and Φ represents the elevation angle.
It is desirable to manage with a preferably small number of antennas, i.e., transmitting antennas and receiving antennas, of the antenna array of the radar device. Use of the known MIMO principle combines reception signals of multiple switching states, and thus allows the formation of virtual arrays with an enlarged aperture of a large number of virtual antennas.
The virtual arrays av(θ) are formed by folding the receiving antenna array, which is made up of the receiving antennas, with the transmitting antenna array, which is made up of the transmitting antennas, i.e., by forming all possible products of the one-way antenna diagrams atx(θ) of the transmitting antennas with the one-way antenna diagrams arx(θ) of the receiving antennas:av(θ)=atx(θ)⊗arx(θ),where ⊗ symbolizes the Kronecker product, i.e., forms all possible products of the elements of vectors atx(θ) and arx(θ).
Conventional radar devices require phase coherence between various switching and transmission states of the transmitting antennas. For time multiplex MIMO, objects which are moved relative to the radar device result in a phase shift between transmission states, for which reason compensation methods for compensating for the phase shift are used. Errors in the speed estimation result in residual phase shifts which have an adverse effect on the subsequent position angle determination, in particular if radar signals having relatively slow FMCW ramps are emitted.